This invention relates to the bleaching of wood pulp and, more particularly, relates to an improvement in the bleaching of wood pulp with hydrogen or sodium peroxide. The term "bleaching" used herein refers to the brightening of wood pulp by decolorization. All brightnesses reported herein were measured using a TECHNIBRITE.TM. brightness meter.
Conventional peroxide bleaching of wood pulp is done at consistencies ranging from 10 to 25%. The bleach liquor is mixed directly with the pulp in mixers requiring a large energy input. The consistency is usually kept as high as possible since it has been found that the higher the consistency the higher the brightness gain, at least up to 35% in the laboratory. However, when the consistency is raised above about 25% in the mill it becomes very difficult to achieve proper mixing of the liquor with the pulp and bleaching efficiency accordingly either levels off or decreases. It is stated by Andrews and Singh in The Bleaching of Pulp by Tappi Press (third edition), that the improvement in brightness between 12% and 25% consistency amounted to only one brightness point.
In 1968 Nardi and Adami received Canadian patent No. 798,620 which discloses a process in which pulp is fragmented in a gaseous environment at consistencies above 30%. Atomized bleach liquor is then sprayed into the pulp to achieve uniform mixing. However, this technology has not been applied commercially.
Lindahl et al. U.S. Pat. No. 4,160,693 granted July 10, 1979 discloses a process for the bleaching of cellulose pulp in which inter alia bleach liquor is mixed with pulp having a reduced consistency at a temperature within the temperature range of 30.degree. to 105.degree. C.
The major difficulty with high consistency peroxide bleaching has always been mixing the liquor with the pulp. When mixing is poor, non-uniform bleaching results, and the final brightness gain is less than anticipated. This results in a practical maximum bleaching consistency of about 25% and, as a consequence, also necessitates higher chemical costs for uniform peroxide bleaching.